Frequency modulator

ABSTRACT

A frequency modulator ( 50, 150 ) for modulating a carrier signal according to a modulation data signal to provide a modulated output signal (RFout), comprises a reference signal generator ( 54, 154 ) coupled to receive the modulation data signal for performing a low frequency modulation process and for generating a reference signal modulated according to the modulation data signal, and a main synthesizer ( 52, 152 ) coupled to receive the modulated reference signal and the modulation data signal for performing a high frequency modulation process and for providing the modulated output signal at an output. Preferably, the modulated reference signal has a first modulation gain (Kr) and the modulated output signal has a modulation gain (KV) which is substantially proportional to the first modulation gain (Kr).

FIELD OF THE INVENTION

This invention relates to a frequency modulator and a transmitter andtransceiver incorporating a frequency modulator for a digital radiocommunication device.

BACKGROUND OF THE INVENTION

Many digital radio communication systems, such as cellular, cordless anddata transmission systems, use FSK, GFSK or GMSK modulation techniques.These types of modulation techniques are in fact simply frequencymodulation (FM) with the radio frequency (RF) signal envelope constant.

Since there is no amplitude modulation (AM) involved in these types ofmodulation, the voltage controlled oscillator (VCO) frequency of thetransmitters can be directly modulated by the baseband signal, as istypical in regular analog FM transmitters, such as in analog cellularsystems. Significant cost reductions can be obtained by directlymodulating the VCO frequency so that such arrangements are particularlydesirable in digital applications where low cost is a strategic factor.For example, the overall cost of a digital solution such as DECT must bevery low to be competitive with well known analog systems such as CT0.

For analog systems, the voice spectrum, 300 to 3000 Hz or more, iscompatible with the lock-up time of the PLL synthesizer when themodulation is applied to the VCO. The lock-up time is generally neverless than 5 or 6 ms. However, when applying this low cost technique indigital systems, the baseband signal, which modulates the VCO, is thefiltered No-Return-to-Zero (NRZ) data stream coming from the logicsection of the transmitter device. This baseband signal has a very lowfrequency content in its spectrum (from a few Hz). Since the requiredlock-up time for the PLL synthesizer is around few microseconds, it isnot compatible with the spectrum of the baseband signal. The lowfrequencies of the modulation are considered like VCO drifts and so arecorrected by the PLL loop. This incompatibility problem means thateither the low frequency content of the baseband signal is lost or thespeed of the PLL loop, which determines the speed for channel selection,is compromised.

Thus, before this low cost technique can be advantageously applied indigital systems, the problem of ensuring compatibility with the PLLsynthesizer lock-up time needs to be resolved and further requirementshave to be considered.

For example, in order to avoid any inter symbol interferences whichcould corrupt the eyes pattern and degrade the Bit Error Rate of thetransmission in the digital system, the group delay on the modulationpath has to be kept constant in all the spectrum of the baseband signal.

Furthermore, in order to meet the applicable radio specifications, thespectral purity of the RF signal source to be transmitted should bemaintained as near the carrier signal as possible for phase noise andmodulation accuracy, and as far from the carrier signal as possible forharmonics, noise floor, discrete spurious signals. Should this lastrequirement be sufficiently met, filters and duplexers would no longerbe needed which would provide a drastic reduction in cost of thetransmitter. Being able to reduce the filters and duplexers issignificant in keeping the overall cost of the digital system down whichimproves the competitiveness of the digital solution compared to theanalog solution.

Several modulation techniques which modulate the VCO in a digital systemare known.

For example an I/Q modulator has been used in a direct or heterodynearchitecture. This type of modulator is driven by two RF signals inquadrature coming from an external oscillator which is tuned on thetransmitted frequency (direct architecture) or on an intermediatefrequency (heterodyne architecture) and by two complex baseband signals,calculated from the data stream (I and Q signals). With sucharchitectures, the RF oscillator(s) or synthesizer(s) are independent ofthe modulation process so that the lock-up time can be chosenindependently from the baseband signal spectrum. However, sucharchitectures are complex and require an expensive ‘I/Q’ interface inthe baseband which can be justified for very accurate GMSK systems, likeGSM, but not in standards like DECT or CT2, where the modulation indexcan vary by large amounts and where cost is strategic. A furtherdisadvantage with these techniques it that they are noisy and so do notallow for the filters and duplexers to be removed.

A different but well known technique uses a heterodyne arrangement witha fixed modulated local oscillator. A channel synthesizer which meetsthe lock-up time requirements is mixed with a crystal oscillator or avery slow synthesizer able to handle the modulation. This type ofarrangement is very efficient and robust and allows zero blind slotsbecause the channel synthesizer is fast enough. In other words, withthis type of arrangement every slot can be used. However, it alwaysgenerates many mixing products out of the bandwidth of the signal to betransmitted, which have to be strongly filtered, increasing the cost ofthe transmit path.

Another technique called the ‘open loop’ technique has been used withcordless standards such as DECT or CT2. As mentioned above, a problemwith applying the modulation to the VCO is that the low frequency partof the FM modulation will be corrected by the loop of the synthesizer asit does for frequency drift of the VCO. In order to avoid this, the openloop technique opens the loop during a transmission slot so that the VCOis working in a free-run mode without any feedback and can be modulatedwithout any correction from the loop. After the transmission slot, theloop is closed again and the VCO resynchronized. Resynchronization ishowever difficult to achieve. In fact, it is so difficult to achieve,that the VCO must typically work at half the final frequency and must befollowed by a frequency doubler to be properly buffered againsttransients and frequency drifts. This requires many filters whichincreases the overall cost of the transmitter device. A furtherdisadvantage of the open loop technique is that it does not allow forzero blind slots because of the frequency drift due to the free-runmode.

In addition, the phase detector of the synthesizer for the open looptechnique is very hard to design because of the low leakage currentrequired.

In fact, the open loop technique cannot be used in ‘high-end’ standards,such as GSM or MOBITEX, where parasitic drifts due to the free-run modeperiods are not allowed by the respective specifications. The open looptechnique can be implemented for standards such as DECT and CT2, but dueto its difficult resynchronisation, only half of the available slots canbe used. This is a problem for public base stations or for rangeimprovement.

There is therefore a need to provide an improved low cost frequencymodulator for a digital radio communication device which overcomes theabove described problems.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a frequencymodulator for modulating a carrier signal according to a modulation datasignal to provide a modulated output signal, the frequency modulatorcomprising:

a reference signal generator coupled to receive the modulation datasignal for generating a reference signal modulated according to themodulation data signal; and

a main synthesizer coupled to receive the modulated reference signal andthe modulation data signal for providing the modulated output signal atan output, and wherein the reference signal generator comprises anauxiliary synthesizer having a lock-up time which is substantiallygreater than that of the main synthesizer, the auxiliary synthesizercomprising:

an auxiliary phase detector having a first input for receiving a fixedreference signal having a fixed frequency, a second input and an outputfor providing an error signal representing the difference in phasebetween the fixed reference signal and a signal at the second input; and

an auxiliary VCO coupled to receive the error signal and the modulationdata signal for generating the modulated reference signal at an output,the output of the auxiliary VCO being coupled to the main synthesizerand to the second input of the auxiliary phase detector.

Thus, by modulating the reference signal supplied to the mainsynthesizer with the modulation data signal and by applying themodulation data signal to the main synthesizer, the present inventionensures that the modulation process to provide the modulated outputsignal is independent of the speed of the main synthesizer. This meansthat the low frequency components of the modulation data signal are notlost nor is the speed of the main synthesizer compromised.

Preferably, the modulated reference signal has a first modulation gainand the modulated output signal has a modulation gain which issubstantially proportional to the first modulation gain. This results inthe modulated output signal RFout having a response which is flat inamplitude and group delay, and independent of modulating frequency.

In a preferred arrangement, the main synthesizer for generating themodulated output signal and for controlling the frequency of the carriersignal, comprises:

a phase detector having a first input for receiving the modulatedreference signal, a second input and an output for providing an errorsignal representing the difference in phase between the modulatedreference signal and a signal at the second input;

a VCO coupled to receive the error signal and the modulation data signalfor generating the modulated output signal at an output of the VCO; and

a variable divider coupled between the output of the VCO and the secondinput of the phase detector for dividing the modulated output signal bya selectable value so as to vary the frequency of the carrier signal.With such an arrangement, the first modulation gain is preferablyarranged to be substantially equal to the gain of the modulated outputsignal divided by the selectable value of the variable divider.

BRIEF DESCRIPTION OF THE DRAWINGS

Two frequency modulators in accordance with the present invention, atransceiver and a transmitter, both incorporating frequency modulatorsin accordance with the invention, will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a block schematic diagram of a transceiver incorporating afrequency modulator in accordance with the present invention;

FIG. 2 is a block schematic diagram of a frequency modulator;

FIG. 3 is a block schematic diagram of a first frequency modulator inaccordance with the present invention;

FIG. 4 is a block schematic diagram of a second frequency modulator inaccordance with the present invention;

FIG. 5 is a schematic diagram showing the frequency components of a PLLloop in which the VCO is modulated;

FIG. 6 is a schematic diagram showing the frequency components of a PLLloop in which the reference signal is modulated; and

FIG. 7 is a schematic diagram showing the frequency components of a PLLloop in which the VCO and the reference signal is modulated by themodulation data signal in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to FIG. 1, part 2 of a transceiver deviceincorporating a frequency modulator 3 in accordance with the presentinvention is shown comprising a transmit path TX and a receive path RXcoupled via a switch 4 and a low-pass filter 8 to an antenna 6.

The frequency modulator 3 has an input 5 for receiving a modulation datasignal which is generated from a data stream using a modulationtechnique, such as FSK, GFSK or GMSK, as is well known in the art. Thedata stream may include voice information. For GMSK modulation, themodulation data signal or baseband signal is an analog voltage generatedby a GMSK analog filter (not shown). The frequency modulator 3 generatesa carrier signal having a selectable frequency and modulates the carriersignal using the modulation data signal to provide a modulated outputsignal RFout at an output. The modulated output signal RFout is thenamplified by the RF power amplifier 10 before being coupled to theantenna 6. The structure and function of the frequency modulator 3 willbe described in more detail below.

The receive path RX includes a first bandpass filter 12, a low noiseamplifier (LNA) 14, a second bandpass filter 16, a mixer 18 and an IFfilter 20. The mixer 18 mixes the RF signal received at the antenna 6with a reference signal which may be generated by a local oscillator(not shown) or by the frequency modulator 3 as shown in FIG. 1 so as toproduce an intermediate frequency signal IF. Using the signal generatedby the frequency modulator 3 avoids the need for an additional localoscillator.

Although the frequency modulator 3 in accordance with the presentinvention has been shown in FIG. 1 as part of a transceiver, it will beappreciated that the frequency modulator 3 in accordance with thepresent invention may be used in a transmitter.

Referring now firstly to FIG. 2, a frequency modulator 50 comprises amain synthesizer 52 and a reference signal generator 54. The modulationdata signal received at an input 51 of the frequency modulator 50 iscoupled to the main synthesizer 52 and the reference signal generator54. The main synthesizer 52 provides at an output the modulated outputsignal RFout and the reference signal generator 54 generates a modulatedreference signal Fr which is supplied to the main synthesizer 52.

The reference signal generator 54 comprises a crystal oscillator 64 anda variable gain control 66. The modulation data signal is applieddirectly to the crystal oscillator so as to provide the modulatedreference signal Fr. The variable gain control 66 is set so that themodulated reference signal Fr has a first modulation gain Kr.

The main synthesizer 52 comprises a phase detector 56, a loop filter 58,voltage controlled oscillator (VCO) 60 and a variable divider 62arranged in a typical phase-locked loop (PLL) arrangement. The variabledivider 62 divides the modulated output signal RFout at an output of theVCO 60 by a selectable value n so as to provide a divided modulatedoutput signal. The selectable value n determines the frequency of thecarrier signal which is generated by the VCO 60. In other words, thevariable divider 62 is the channel selector of the main synthesizer 52.

The phase detector 56 has a first input for receiving the modulatedreference signal Fr, a second input for receiving the divided modulatedoutput signal and an output for providing an error signal representingthe difference in phase between the modulated reference signal and thedivided modulated output signal. The error signal is then filtered bythe loop filter 58 before being applied to the VCO 60.

The filtered error signal, which is a voltage signal, is applieddirectly to the VCO 60 so as to generate a carrier signal whosefrequency depends on the filtered error signal.

A disadvantage of such a frequency modulator 50 is that the crystaloscillator 64 itself is modulated and so cannot be used for other partsof the transceiver device. A device incorporating the frequencymodulator 50 would therefore require additional crystal oscillatorswhich would increase the cost of such a device. A further disadvantageof the frequency modulator 50 is that for wideband modulation (i.e. highdata rates), the technique of modulating the crystal oscillator cannotbe used.

Referring now to FIG. 3, a first frequency modulator 150 in accordancewith a first embodiment of the present invention comprises a mainsynthesizer 152 and a reference signal generator 154. The modulationdata signal received at an input 151 of the frequency modulator 150 iscoupled to the main synthesizer 152 and the reference signal generator154. The main synthesizer 152 provides at an output the modulated outputsignal RFout and the reference signal generator 154 generates amodulated reference signal Fr which is supplied to the main synthesizer152.

The reference signal generator 154 comprises an auxiliary synthesizer155 coupled to a crystal oscillator 164 and to receive the modulationdata signal at the input 151 such that the modulation data signalmodulates an auxiliary VCO 170 of the auxiliary synthesizer 155. Theoutput of the auxiliary VCO 170 provides the modulated reference signalto the main synthesizer 152.

The main synthesizer 152 comprises a phase detector 156, a loop filter158, voltage controlled oscillator (VCO) 160 and a variable divider 162arranged in a typical phase-locked loop (PLL) arrangement. The variabledivider 162 divides the modulated output signal RFout at an output ofthe VCO 160 by a selectable value n so as to provide a divided modulatedoutput signal. The selectable value n determines the frequency of thecarrier signal which is generated by the VCO 160. In other words, thevariable divider 162 is the channel selector of the main synthesizer152.

The phase detector 156 has a first input for receiving the modulatedreference signal Fr, a second input for receiving the divided modulatedoutput signal and an output for providing an error signal representingthe difference in phase between the modulated reference signal and thedivided modulated output signal. The error signal is then filtered bythe loop filter 58 before being applied to the VCO 160.

Normally, the filtered error signal, which is a voltage signal, isapplied directly to the VCO 160 so as to generate a carrier signal whosefrequency depends on the filtered error signal. However, in thepreferred embodiment, the modulation data signal, in addition to thefiltered error signal, is applied to the VCO 160, for example bycombining the modulation data signal, which as mentioned above is ananalog baseband voltage, with the filtered error signal, such that theanalog baseband voltage and the error voltage determines the output ofthe VCO 160.

In order to apply the filtered error signal and the modulation datasignal to the VCO 160, the VCO may have two control inputs or onecontrol input. Preferably, the filtered error signal and the modulationdata signal are combined by applying the modulation data signal to amodulation varicap diode of the VCO 160 and the filtered error signal toa tuning diode of the VCO 160. In a further embodiment, both themodulation data signal and the filtered error signal may be applied tothe tuning varicap diode, such as diode 168.

By modulating the VCO 170 rather than the crystal oscillator, the firstembodiment has an advantage in that the reference frequency signal Fr′generated by the crystal oscillator 164 can be re-used for differentpurposes within the transceiver. This is not possible with the frequencymodulator 50 since the crystal reference is modulated directly.Moreover, in same applications, particularly those where the modulationindex is high such as in DECT systems, there can be problems inmodulating the frequency of the crystal oscillator. Using an auxiliarysynthesizer to make a copy of the signal from the crystal oscillator 164avoids this problem.

The auxiliary synthesizer 154 comprises an auxiliary phase detector 174,an auxiliary loop filter 176, an auxiliary VCO 170 and a divider 172arranged in a typical phase-locked loop (PLL) arrangement. The divider172 divides the modulated reference signal Fr at an output of theauxiliary VCO 170 by a fixed value so as to provide a divided modulatedreference signal.

The auxiliary phase detector 174 has a first input for receiving thereference signal Fr′ generated by the crystal oscillator 164, a secondinput for receiving the divided modulated reference signal and an outputfor providing an error signal representing the difference in phasebetween the reference signal Fr′ and the divided modulated referencesignal. The error signal is then filtered by the auxiliary loop filter176 before being applied to the auxiliary VCO 170 together with themodulation data signal. These signals are combined in similar manner tothat described above for VCO 160. For example, the modulation datasignal and filtered error signal are both applied to a tuning varicapdiode 178 of the VCO 170. The modulation data signal is coupled to theauxiliary synthesizer 155 via a variable gain control 166 so that theVCO 170 is modulated with a first modulation gain Kr to provide themodulated reference signal Fr at the output of the VCO 170.

The modulation gain KV of the VCO 160 is preferably adjustable by havingan attenuator (not shown) coupled between the input 151 of the secondfrequency modulator 150 and an input of the VCO.

Referring now also to FIG. 4, a second frequency modulator 250 inaccordance with a second embodiment of the present invention comprises amain synthesizer 252 and a reference signal generator 254. A data streamreceived at an input 251 of the second frequency modulator 250 iscoupled to the main synthesizer 252 and the reference signal generator254. In the embodiment shown in FIG. 4, GMSK modulation is used. Thedata stream received at input 251 is thus filtered in a GMSK analogfilter 253 before being coupled to the main synthesizer 252 as amodulation data signal. The main synthesizer 252 provides at an outputthe modulated output signal RFout and the reference signal generator 254generates a modulated reference signal Fr which is supplied to the mainsynthesizer 252.

The reference signal generator 254 in accordance with the secondembodiment comprises a direct digital synthesizer (DDS), such as theAD9830 DDS supplied by Analog Devices. The DDS 254 has a digital inputcoupled directly to the data stream before filtering which allows forthe reference frequency generated to be digitally modulated by themodulation data signal and an output for providing the modulatedreference signal Fr to the main synthesizer 252. This modulation in theDDS 254 can be performed by an auxiliary look-up table 255, computing onN bits, for each clock period, the modulation trajectory, N giving theaccuracy of the process (the greater is N, the greater is the accuracy).A channel or a sub-channel increment can be performed through theregular N bits adressing bus of the main look-up table 257, afteraddition to the modulation N bits result, in order to vary slightly ifneeded, the centre of the reference frequency Fr, and consequently, thecentre vlaue of the modulated output signal Rfout. The operation of theDDS is well known in the art.

The main synthesizer 252 comprises a phase detector 256, a loop filter258, voltage controlled oscillator (VCO) 260 and a fixed divider 263arranged in a typical phase-locked loop (PLL) arrangement.

The operation of the main synthesizer 252 is the same as described abovewith reference to FIG. 3. Like components to those of FIG. 3 arereference by the same reference numeral plus the number one hundred.

The basic principle of operation of the frequency modulator inaccordance with the present invention is to apply the modulation datasignal to the VCO of the main synthesizer and also to the referencesignal with a correct level balance between the two paths.

In order to understand completely this basic principle, a theoreticalanalysis will now be given with reference to FIGS. 5-7. FIG. 5 is aschematic diagram showing the frequency components of a PLL loop inwhich the VCO is modulated only, FIG. 6 is a schematic diagram showingthe frequency components of a PLL loop in which the reference signal ismodulated only and FIG. 7 is a schematic diagram showing the frequencycomponents of the PLL loop in which the VCO and the reference signal ismodulated by the modulation data signal in accordance with the presentinvention.

Referring firstly to FIG. 5, the voltage signal e which is applied tothe varicap diode of a VCO 600 to generate a modulated output signal OUTis given by the following equation: $\begin{matrix}{e = {{A + {em}} = {\frac{e\left( {{KV}*K\quad Ø*{F(p)}} \right)}{np} + {em}}}} & (1)\end{matrix}$

where em is the modulation data signal

Fconst is a constant reference signal

KV is the modulation gain of the VCO 600 in Hz per Volts

KØ is the gain of the phase detector 560 in Volts per Radians

F(p) is the transfer function of the loop filter 580 (low pass filter)

n is the divide ratio of the variable divider 620

From equation 1, the transfer function for the modulation on the VCOloop is: $\begin{matrix}{\frac{e}{em} = \frac{1}{1 - \frac{\left( {{KV}*K\quad Ø*{F(p)}} \right)}{np}}} & (2)\end{matrix}$

This transfer function represents a high pass behaviour.

Referring now also to FIG. 6, for a modulated reference signal Fmod, theerror signal Σ at the output of a phase detector 560 is given by thefollowing equation: $\begin{matrix}{\Sigma = {\frac{Fmod}{p} - {e\frac{KV}{np}}}} & (3)\end{matrix}$

The voltage signal e is given by:

e=−KØ*F(p)*Σ  (4)

By combining equations (3) and (4), the transfer function for themodulation on the reference signal loop is: $\begin{matrix}{\frac{e}{Fmod} = \frac{n*K\quad Ø*{F(p)}}{\left( {K\quad Ø*{F(p)}*{KV}} \right) - {np}}} & (5)\end{matrix}$

This represents a low pass behaviour.

Referring now to FIG. 7, the voltage signal e which is applied to thevaricap diode of the VCO 160 of FIG. 3 or the VCO 260 of FIG. 4 togenerate the modulated output signal RFout is given by the followingequation:

e=em−KØ*F(p)*Σ(6)

Since the error signal Σ at the output of a phase detector 56 is:$\begin{matrix}{\Sigma = {{{em}\frac{Kr}{p}} - {e\frac{KV}{np}}}} & (7)\end{matrix}$

where em is the modulation data signal

Fr is the modulated reference signal

KV is the modulation gain of the VCO 60, 160 in Hz per Volts

KØ is the gain of the phase detector 56, 156 in Volts per Radians

F(p) is the transfer function of the loop filter 58, 158 (low passfilter)

n is the divide ratio of the variable divider 62, 162

Kr is the first modulation gain of the modulated reference signal Fr

By substituting equation (7) in equation (6), then: $\begin{matrix}{\frac{e}{em} = \frac{1 - \frac{\left( {{Kr}*K\quad Ø*{F(p)}} \right)}{p}}{1 - \frac{\left( {{KV}*K\quad Ø*{F(p)}} \right)}{np}}} & (8)\end{matrix}$

From equation (8),${{{when}\quad {Kr}} = \frac{KV}{n}},{{{then}\quad \frac{e}{em}} = 1}$

Thus, from the above theoretical analysis when the first modulation gainKr of the modulated reference signal Fr is equal to the modulation gainKV of the VCO of the main synthesizer divided by n, there is no low passand high pass behaviour. In practice, it has been found thatsubstantially the same effect can be achieved when the first modulationgain Kr of the modulated reference signal Fr is close to the modulationgain KV of the VCO of the main synthesizer divided by n; that is, whenthe first modulation gain Kr of the modulated reference signal Fr isproportional to the modulation gain KV.

Though the first modulation gain Kr is adjusted with the selectablevalue n and hence with the carrier frequency, the system is broadbandenough to require only one trimming point within a 100 Mhz bandwidth ormore at 2 GHz.

As mentioned above, the principle of operation of the frequencymodulator in accordance with the present invention is to apply themodulation data signal to the VCO of the main synthesizer and also tothe reference signal with a correct level balance between the two paths.Correct balance occurs when the values of the modulation gains Kr and KVare selected so that the first modulation gain Kr of the modulatedreference signal Fr is substantially proportional to the modulation gainKV of the VCO. This results in the modulated output signal RFout havinga response which is flat in amplitude and group delay, and independentof modulating frequency.

For the first frequency modulator 150, it is also necessary to ensurethat the bandwidth of the auxiliary synthesizer 155 is substantiallyless than the bandwidth of the main synthesizer 152 or in other words,that the lock-up time of the auxiliary synthesizer 155 is substantiallygreater than that of the main synthesizer 152. The reference signalgenerator 154 then performs a low frequency modulation process, and theVCO of a main synthesizer 152 is involved in the high frequencymodulation process with a result that the combination covers the wholebandwidth of the modulation data signal. Since the low frequencycomponents of the modulation data signal modulate the reference signals,these components are not lost in the main synthesizer's PLL loop as theywould be if only the main VCO was modulated. These low frequencycomponents are then combined with the high frequency components of themodulation data signal which components are outside the bandwidth of themain synthesizer loop so as to provide the modulated output signal.

By utilising an auxiliary synthesizer 154 having a greater lock-up timethan the main synthesizer 152 and by modulating the auxiliary VCO 170 bythe modulation data signal, the main synthesizer 152 can be fast enoughto be used as a channel selector and meet the lock-up time specificationas required for the high frequency components.

Thus, from the above analysis it is clear that by modulating thereference signal supplied to the main synthesizer and the VCO of themain synthesizer with the modulation data signal, the present inventionensures that the modulation is independent of the speed of the mainsynthesizer. This means that the low frequency components of themodulation data signal are not lost nor is the speed of the mainsynthesizer compromised. Moreover, since the present invention ensuresthat the modulation is independent of the speed of the main synthesizer,there is substantially no limitation on the speed for the channelselection.

Since the present invention applies the modulation data signal directlyto the main VCO 160, 260 which is coupled to the antenna via a poweramplifier but not via mixers or other noisy components as with the priorart systems, the present invention provides a low noise and directsynthesis process which means that filters and duplexers are no longerneeded in the transmit path and a lower cost power amplifier can beused. This represents a drastic cost reduction compared to known digitalsystems, some of which are described above in the introduction.

An additional advantage of the frequency modulator in accordance withthe present invention, as can be seen from the first 150 and second 250frequency modulators described above, is that it can be realised withlow cost component blocks.

A further advantage is that the main synthesizer is always locked whichmeans that every slot can be used in TDMA systems allowing datatransmission or real range improvement.

Since the present invention ensures that the speed of the mainsynthesizer of the frequency modulator is optimised, the same mainsynthesizer can be used for the receive path RX in a transceiver. Asmentioned above, such an arrangement avoids the need for an additionalsynthesizer in the receive path.

Although the present invention has been described above with referenceto digital radio communication devices, the principle of the presentinvention could also be applied to analog radio communication devices.

What is claimed is:
 1. A frequency modulator for modulating a carriersignal according to a modulation data signal to provide a modulatedoutput signal, the frequency modulator comprising: a reference signalgenerator coupled to receive the modulation data signal for generating areference signal modulated according to the modulation data signal; anda main synthesizer coupled to receive the modulated reference signal andthe modulation data signal for providing the modulated output signal atan output, and wherein the reference signal generator comprises anauxiliary synthesizer for performing a low frequency modulation processand having a lock-up time which is substantially greater than that ofthe main synthesizer, the auxiliary synthesizer comprising: an auxiliaryphase detector having a first input for receiving a fixed referencesignal having a fixed frequency, a second input and an output forproviding an error signal representing the difference in phase betweenthe fixed reference signal and a signal at the second input; and anauxiliary VCO coupled to receive the error signal and the modulationdata signal for generating the modulated reference signal at an output,the output of the auxiliary VCO being coupled to the main synthesizerand to the second input of the auxiliary phase detector.
 2. A frequencymodulator according to claim 1 wherein the main synthesizer forgenerating the modulated output signal and for controlling the frequencyof the carrier signal, comprises: a phase detector having a first inputfor receiving the modulated reference signal, a second input and anoutput for providing an error signal representing the difference inphase between the modulated reference signal and a signal at the secondinput; a VCO coupled to receive the error signal and the modulation datasignal for generating the modulated output signal at an output of theVCO; and a variable divider coupled between the output of the VCO andthe second input of the phase detector for dividing the modulated outputsignal by a selectable value so as to vary the frequency of the carriersignal.
 3. A frequency modulator according to claim 2 wherein themodulated reference signal has a first modulation gain and the modulatedoutput signal has a modulation gain and wherein the first modulationgain is substantially equal to the gain of the modulated output signaldivided by the selectable value of the variable divider.
 4. A frequencymodulator according to claim 2, wherein the VCO comprises a varicaptuning diode, the modulation data signal and the error signal beingapplied to the varicap tuning diode.
 5. A frequency modulator accordingto claim 2, wherein the VCO comprises a varicap tuning diode and avaricap modulating diode, the modulation data signal being applied tothe varicap modulating diode and the error signal being applied to thevaricap tuning diode.
 6. A frequency modulator according to claim 1wherein the modulated reference signal has a first modulation gain andthe modulated output signal has a modulation gain which is substantiallyproportional to the first modulation gain.
 7. A frequency modulatoraccording to claim 6 wherein the gain of the modulated output signal isadjustable.
 8. A frequency modulator according to claim 6 furthercomprising an attenuator coupled between an input for receiving themodulation data signal and the main sythesizer for adjusting the gain ofthe modulated output signal.
 9. A frequency modulator according to claim1 wherein the auxiliary synthesizer further comprises a divider coupledbetween the output of the auxiliary VCO and the second input of theauxiliary phase detector for dividing the modulated reference signal bya fixed value.
 10. A frequency modulator according to claim 1 whereinthe reference signal generator further comprises a variable gain controlfor controlling the first modulation gain of the modulated referencesignal.
 11. A frequency modulator for modulating a carrier signalaccording to a modulation data signal to provide a modulated outputsignal, the frequency modulator comprising: a reference signal generatorcoupled to receive the modulation data signal for generating a referencesignal modulated according to the modulation data signal; and a mainsynthesizer coupled to receive the modulated reference signal and themodulation data signal for performing a high frequency modulationprocess and for providing the modulated output signal at an output, andwherein the reference signal generator comprises a direct digitalsynthesizer for performing a low frequency modulation process and havingan input for receiving the modulation data signal and an output forproviding the modulated reference signal, the modulated reference signalhaving a frequency which is dependent on a channel select signal.
 12. Atransmitter for a digital radio communication device having a transmitpath comprising: a frequency modulator for modulating a carrier signalaccording to a modulation data signal to provide a modulated outputsignal, the frequency modulator comprising: a reference signal generatorcoupled to receive the modulation data signal for generating a referencesignal modulated according to the modulation data signal; and a mainsynthesizer coupled to receive the modulated reference signal and themodulation data signal for performing a high frequency modulationprocess and for providing the modulated output signal at an output, andwherein the reference signal generator comprises an auxiliarysynthesizer for performing a low frequency modulation process and havinga lock-up time which is substantially greater than that of the mainsynthesizer, the auxiliary synthesizer comprising: an auxiliary phasedetector having a first input for receiving a fixed reference signalhaving a fixed frequency, a second input and an output for providing anerror signal representing the difference in phase between the fixedreference signal and a signal at the second input; and an auxiliary VCOcoupled to receive the error signal and the modulation data signal forgenerating the modulated reference signal at an output, the output ofthe auxiliary VCO being coupled to the main synthesizer and to thesecond input of the auxiliary phase detector; a power amplifier foramplifying the modulated output signal; and an antenna coupled to thepower amplifier.
 13. A frequency modulator according to claim 12 whereinthe main synthesizer for generating the modulated output signal and forcontrolling the frequency of the carrier signal, comprises: a phasedetector having a first input for receiving the modulated referencesignal, a second input and an output for providing an error signalrepresenting the difference in phase between the modulated referencesignal and a signal at the second input; a VCO coupled to receive theerror signal and the modulation data signal for generating the modulatedoutput signal at an output of the VCO; and a variable divider coupledbetween the output of the VCO and the second input of the phase detectorfor dividing the modulated output signal by a selectable value so as tovary the frequency of the carrier signal.
 14. A frequency modulatoraccording to claim 13 wherein the modulated reference signal has a firstmodulation gain and the modulated output signal has a modulation gainand wherein the first modulation gain is substantially equal to the gainof the modulated output signal divided by the selectable value of thevariable divider.
 15. A frequency modulator according to claim 13,wherein the VCO comprises a varicap tuning diode, the modulation datasignal and the error signal being applied to the varicap tuning diode.16. A frequency modulator according to claim 13, wherein the VCOcomprises a varicap tuning diode and a varicap modulating diode, themodulation data signal being applied to the varicap modulating diode andthe error signal being applied to the varicap tuning diode.
 17. Afrequency modulator according to claim 12 wherein the modulatedreference signal has a first modulation gain and the modulated outputsignal has a modulation gain which is substantially proportional to thefirst modulation gain.
 18. A frequency modulator according to claim 17wherein the gain of the modulated output signal is adjustable.
 19. Afrequency modulator according to claim 17 further comprising anattenuator coupled between an input for receiving the modulation datasignal and the main synthesizer for adjusting the gain of the modulatedoutput signal.
 20. A transceiver for a digital radio communicationdevice having a transmit path and a receive path, the transmit pathcomprising: a frequency modulator for modulating a carrier signalaccording to a modulation data signal to provide a modulated outputsignal, the frequency modulator comprising: a reference signal generatorcoupled to receive the modulation data signal for generating a referencesignal modulated according to the modulation data signal; and a mainsynthesizer coupled to receive the modulated reference signal and themodulation data signal for performing a high frequency modulationprocess and for providing the modulated output signal at an output, andwherein the reference signal generator comprises an auxiliarysynthesizer for performing a low frequency modulation process and havinga lock-up time which is substantially greater than that of the mainsynthesizer, the auxiliary synthesizer comprising: an auxiliary phasedetector having a first input for receiving a fixed reference signalhaving a fixed frequency, a second input and an output for providing anerror signal representing the difference in phase between the fixedreference signal and a signal at the second input; and an auxiliary VCOcoupled to receive the error signal and the modulation data signal forgenerating the modulated reference signal at an output, the output ofthe auxiliary VCO being coupled to the main synthesizer and to thesecond input of the auxiliary phase detector; a power amplifier foramplifying the modulated output signal; and an antenna for coupling tothe power amplifier.
 21. A transmitter for a digital radio communicationdevice having a transmit path comprising: a frequency modulator formodulating a carrier signal according to a modulation data signal toprovide a modulated output signal, the frequency modulator comprising: areference signal generator coupled to receive the modulation data signalfor generating a reference signal modulated according to the modulationdata signal; and a main synthesizer coupled to receive the modulatedreference signal and the modulation data signal for performing a highfrequency modulation process and for providing the modulated outputsignal at an output, and wherein the reference signal generatorcomprises a direct digital synthesizer for performing a low frequencymodulation process and having an input for receiving the modulation datasignal and an output for providing the modulated reference signal, themodulated reference signal having a frequency which is dependent on achannel select signal; and a power amplifier for amplifying themodulated output signal; and an antenna coupled to the power amplifier.